"Power-line carriers" are well known in the field of power system communications. The principal elements of such power-line carriers are transmitting and receiving terminals, which include one or more line traps, one or more coupling capacitors, as well as tuning and coupling equipment. Detailed information regarding the description and typical composition of conventional power line carriers may be found in Fundamentals Handbook of Electrical and Computer Engineering, Volume II: Communication, Control, Devices, and Systems, John Wiley & Sons, 1983, pp 617-627, the contents of which are incorporated herein by reference.
A significant problem associated with prior art power-line carriers is their requirement for one or more line traps, one or more capacitors, one or more coupling transformers or one or more carrier frequency hybrid circuits and frequency connection cables. The new power-line carrier system presents a solution to the fundamental problem of matching the electrical line characteristic impedance with the line coupler. The novel signal coupler design is easily adaptable for operation on distribution and low voltage lines.
All traditional couplers incorporate a ferrite or iron core transformer which causes signal distortion due to the non-linear phase characteristic of the transfer function between the transmit coupler and the receive coupler. The distortion is created by the presence of magnetic core material which exhibits hysteresis. For distribution power-line carriers, the distortion is particularly severe because the signal must propagate through three such non-linear devices, the distribution transformer and two power-line couplers, that use ferrite core transformers. The distortion leads to envelope delay distortion which limits communication speeds.
A line with characteristic impedance Zo is ideally matched by terminations equal to Zo at both ends. Since Zo is primarily resistive at the frequencies of interest, the input impedance of the couplers should also be primarily resistive and equal to Zo at the carrier frequencies. A general configuration to achieve this is shown in FIG. 4. It uses a serially connected equivalent capacitor, C.sub.eq, on the primary of a transformer. The design is based on two principles. First, the resonance between the coupling capacitor, C.sub.eq and the primary winding inductance, Li, provides a low resistive impedance at the desired transmit carrier frequency. Second, C.sub.eq has a large enough impedance at 60 Hz to block the line frequency. Note that this approach is not new, however, previous efforts at achieving satisfactory impedance matching encountered problems discussed below.
The major shortcoming of previous designs resulted from the use of ferrite or iron core transformers in the signal couplers. The inductance, L1, is altered to some unknown valve due to the non-linearity of the core. This results in a mistuning of the desired carrier frequency. Also, the impedance of the primary winding at the desired carrier frequency is no longer purely resistive. This may lead to a mismatch with respect to the line characteristic impedance. In recognition of this fact, other designs (FIGS. 1, 2) attempt to merely couple the signal onto the power line with a low transceiver input impedance by using a large coupling capacitor (approx. 0.5 uF). This results in a significant coupling loss of up to 20 dB at carrier frequency.
The present invention, characterized in FIG. 4, has two coaxial solenoids or air-coils of different diameter with primary and secondary inductances L1 and L2 respectively. Both L1 and L2 are inductively and capacitively coupled creating an air-core transformer (see FIG. 9A). The air-gap is filled with resin which insulates the AC current from the transceiver. The size of the gap is selected to reduce inductive loading effects from coupler secondary to the primary. Since the coupling capacitor, Ceq, is significantly larger than the static capacitor, Cs the static capacitor (FIG. 20) does not mistune the desired carrier frequency. Inductive loading effects from the secondary to primary of the air-core transformer are minimized at the transmit frequency. The effective transceiver input independence, as seen at the primary, is equal to the resistance of the primary winding (R.sub.t or R.sub.r). This value can be chosen to optimally match the line characteristic independence. When Zo equals the resistance of the primary winding, Rt, of the air-core transformer about 25% of the source power can be coupled into the line through the powerline coupler. Note that Zo varies between 5 and 150 Ohms on distribution lines and 1 and 20 Ohms on 120/240 V network lines depending on loading conditions. Since insertion loss increases rapidly for termination impedances were the primary winding impedance is greater than Zo (as compared to primary winding impedance less than Zo), a prudent design choice is to use a value of primary winding resistance approximately equal to the minimum value of the line characteristic impedance, Zo.
The advantage of an air-core transformer in the novel coupler is exhibited by the frequency response shown in FIG. 5. There is a considerably greater band width around the center frequency when comparing it to the response of a traditional coupler which uses a magnetic-core transformer (FIG. 3).
A significant reduction of 60 Hz harmonics are observed at the secondary side of the novel coupler. This reduction can exceed 20 dB over a wide band. Most noise generated on power lines by AC motors and equipment has a large reactive source impedance. This type of noise experiences significant loss through the novel couplers due to the coupler's low resistive impedance at or around the carrier frequency of the transmission or reception. In contrast, the transfer characteristic of ferrite or iron core couplers typically has a high Q (FIG. 3), which is advantageous in theory for reducing the effects of the harmonics outside the bandwidth, but in actuality constrains the useful transmission bandwidth of the power-line carrier and does not provide noise attenuation inside the bandwidth. The wide bandwidth noise rejection of the novel coupler obviates the need for a sinx/x type receive filter for harmonic rejection. This implies that no separate receiver is required, other than the coupler, for high speed transmission.
Another significant aspect of the design is the phase linearity achieved. The matching of the line impedance and the use of air-core transformers are responsible for the amount of phase linearity achieved. In fact, the phase response of the overall transmission system is linear over a very wide range of frequencies. This implies that almost any desired frequency range can be selected for communication. Also, standing waves are virtually suppressed due to the low resistive matching at both ends of the line. The peak amplitude of the first reflection is around 40 mV, which is small compared to the transmitted signal amplitude of a few volts. Thus, setting the receiver threshold above 40 mV can eliminate any remaining source errors. There is also an elimination of standing waves on the line. This implies that there are no anti-nodes, places where the magnitude of the standing wave is zero and no transmission can occur, at points on the line situated at odd multiples of lambda/4 away from the end of the line.
The best frequency range 120/240 V power lines is 70-160 KHz (this includes LAN operations). For data transmission through power line transformers the optimal frequency to use is the 25-45 KHz band. For very high speed LAN applications a frequency range of 70-480 KHz is appropriate. Finally, the novel coupler of the present invention is equally applicable to any voltage AC, DC, phone, twisted pair or coaxial line.
In view of the above, it is an object of the present invention to provide a power line communications apparatus which utilizes a novel phase shift linear power, phone, twisted pair, and coaxial line coupler for both transmission and reception.
It is a further object of the present invention to provide power-line communication apparatus utilizing novel air-core transformers which can be used for phone line, coaxial, LAN, and power line communication through power line transformers.
It is an additional object of the present invention to provide a power-line communication apparatus in which the primary coil of the transformer resonates with an associated coupling capacitor network in order to achieve resistive matching to approximately the lowest known value of the line characteristic impedance and to maximize stable signal transmission onto the line. This resonation effectively creates a band pass filter at carrier frequency.
It is still a further object of the present invention to provide a communications apparatus in which an air-core transformer has primary and secondary windings in which the ratio of the windings is about 1:1.
It is still yet a further object of the present invention to provide a communications apparatus in which the receiver coupling contains a capacitor network which impedes the 60 Hz high power signal and its harmonics.
It is still yet a further object of the present invention to provide a communications apparatus in which the capacitor network for both transmission and reception include resistors which divide down the AC voltage evenly. The resistors also serve to protect the system against spiking and lightning.
It is still yet a further object of the present invention to provide a communications apparatus which can provide a high bandwidth for the transmission of communications signals at speeds greater than 9600 baud, and at speeds of greater than 1200 baud directly through power line transformers.
It is yet a further object of the present invention to provide a communications apparatus containing a phase shift linear air-core transformer effectively comprising two or more solenoids each having different diameters and coaxially within one another such that an air-gap is created, which is usually filed with resin, and which reduces inductive loading effects from the coupler secondary to primary by using the capacitance created in the air-core transformer.
It is still yet a further object of the present invention to provide an apparatus for power system communications over long distances. Because of the low resistive matching of the coupler to the line characteristic impedance, it eliminates standing waves, which implies that there are no anti-nodes at points on the line situated at odd multiples of lambda/4, (3 lambda/4 etc.) away from the end of the line from which no transmission can occur. The low resistive matching also enables communication over long distances.
It is still yet an additional object of the present apparatus to provide power line communications in which the aircore in the coupling transformer gives negligible pulse dispersion and allows for a low resistive matching at the coupler which significantly reduces the power line noise at the coupler output over a wide bandwidth establishing a stable amplitude transfer function with linear phase characteristic over the transmission line.
It is yet another object of the present invention to provide an apparatus for power line communications in which the coupling capacitor resonates with the primary side of the air-core transformer.
It is still a further object of the present invention to provide a novel air-core transformer coupled with a coupling capacitor which provides resistive matching to both sides of the power line transformer to establish a phase shift linear system over the power line and which reduces coupling losses through the power line transformer.